Presses a key on a focused element.
Special keys like 'Enter', 'Control', [etc](https://code.google.com/p/selenium/wiki/JsonWireProtocol#/session/:sessionId/element/:id/value)
will be replaced with corresponding unicode.
If modifier key is used (Control, Command, Alt, Shift) in array, it will be released afterwards.

```js
I.pressKey('Enter');
I.pressKey(['Control','a']);
```

@param {string|string[]} key key or array of keys to press.
@returns {void} automatically synchronized promise through #recorder
